1. Field of the Invention
The present invention relates broadly to the field of electrical heaters and more particularly to a method of manufacturing electrical heaters which produces high production rates and high reliability.
2. Discussion of Background
Electrical heaters and other heating elements are widely used in many industrial heating devices as well as in household devices such as ovens. U.S. Pat. No. 3,201,738 to Mitoff, for example, discloses a heating element surrounded by a layer of magnesim-oxide, lithium-oxide insulation material and an outer protective jacket. Mitoff is directed to decreasing electrical current leakage between the resistance coils of the heating element via the use of the doped insulation material.
U.S. Pat. No. 3,211,203 to Ragland discloses an improved monolithic cathode-heater having electron tubes and a mount for the tubes. Ragland further discloses a method of making a tube having such a mount. U.S. Pat. No. 3,238,489 to Hay discloses a means for insulating and dispersing the heat of electronic units such as an electrical resistor by providing a housing that will transfer heat to the atmosphere, as well as providing an insulator between the resistor and the housing.
U.S. Pat. No. 4,633,063 to Willis discloses an improved printed circuit heating element useful for the sterile welding of thermoplastic tubes. And U.S. Pat. No. 4,501,951 to Benin et al. discloses an electric heating element for sterilely cutting and welding together thermoplastic tubes.
The method generally known in the art for manufacturing heating elements, such as those disclosed above, includes providing an insulator of some composition, generally ceramic, carefully wrapped with a wire heating element to produce a known watt density. An outer metallic jacket is then formed to conform to the inner core. Before the outer metallic jacket is mated with the insulator, a finely powdered material is usually introduced between the outer shell and the heating element. This material must be positioned to separate the metallic sheath from the wire heating element and to separate the heater elements from each other. This process is particularly difficult in the production process and, thus, reduces the production rates and the production reliability. Thus, there remains a strong need for a practical, economical, reliable method of manufacturing electrical heaters.